Alloy 718, which is an Fe—Ni based heat-resistant superalloy used in gas turbine parts for aircraft and power generation, has been widely used for its excellent mechanical properties. In particular, a high fatigue strength is required for large rotating parts in jet engines and gas turbines. Accordingly, Alloy 718 used for such parts is required to have a further enhanced fatigue strength by evenly micronizing grains. For evenly micronizing grains, a billet is often prepared from an ingot of Alloy 718 and then subjected to hot working in a temperature range of 930 to 1010° C. by making use of the pinning effect of the delta phase to form a fine recrystallized structure, and the billet is then subjected to solution treatment (solid solution heat treatment) and aging, or directly to aging.
However, when carrying out hot working under low strain conditions by, for example, closed die forging or ring rolling, abnormal grain growth (hereinafter referred to as AGG) may occur and grains are rapidly coarsened beyond the pinning of the delta phase during the hot working, cooling after the hot working, or solution treatment after the hot working. When such AGG occurs as shown in FIG. 2, a uniform fine structure is broken, and therefore the fatigue characteristic deteriorates. According to Patent Document 1, an influential factor for preventing AGG is identified and a strain of 0.125 or higher is applied in the entirety of the part so as to avoid AGG.